This invention relates generally to ultra low temperature refrigeration and more specifically to a closed loop, supercritical helium gas refrigeration process and apparatus.
Present technology includes two general methods for the cooling of ultra low temperature devices, such as super conducting magnets, generators, and cryopumps. One method uses liquid helium and allows the helium to boil within the device to maintain a constant temperature. The other method is to force flow a stream of supercritical gas at low temperature through the device. The second method has the advantage of providing greater flexibility of design but is limited by the fact that a temperature rise occurs as the cooling stream flows through the device. This is in contrast to the constant temperature provided by the isothermal boiling of liquid helium. The temperature rise can be reduced by increasing the flow, but the flow required to do so is much higher than can be provided by the conventional helium refrigerator. One approach to provide increased flow has been the use of a circulating loop with a cold circulating pump at liquid helium temperature. This has the disadvantages of considerable complexity plus the need to increase the size of the refrigerator in order to remove the heat generated by the circulating pump.
U.S. Pat. No. 4,161,107 discloses a method of producing low temperatures in which the output of helium gas from a compressor is split into two streams which are stepwise cooled by the return flow. The main stream and at least a portion of the subsidiary stream are liquified and used to sustain a refrigerative load. The liquid evaporates as heat is removed. Such a pool boiling or evaporative technique requires a phase change before being passed through the load and cooling cans surrounding the load to hold the boiling helium. The need for such cooling cans makes the use of this system impractical, or at best, complex where the size and/or geometry of the load requires a series of such cooling cans. With the need for liquid helium refrigerator systems having a large capacity for present as well as future technologies, such as nuclear fusion, there is a need to combine the advantages of increased cooling capacity and the flexibility of design of a supercritical gas refrigeration method with the constant temperature cooling of the evaporative method.